HVAC ContractingService and Maintenance

Checking Indoor Airflow

Indoor airflow in any air conditioning system is critical. Too much air and the customer suffers lack of dehumidification in cooling. Too little air in cooling may freeze the coil causing damage to both indoor coil and outdoor compressor.

Figure 1. Inclined manometer.

Checking airflow is one of the most important checks. Instruments can measure pressure drop across the coil. If pressure drop tables are available, they indicate the cubic feet of air per minute (cfm) moving through the coil. An inclined manometer or Magnehelic gauge are examples of this type of instrument. (See Figure 1.)

A velometer measures the velocity of the air in feet per minute (fpm). If the area of duct is known, the velometer reading times the area of duct (in square feet) calculates the airflow.

There are also simple mathematical formulas that are needed. All of them require measurements be taken. These measurements require a voltmeter, ammeter, and a good temperature tester.

Note: These following procedures to measure airflow can be done with electric resistance heaters.

cfm = volts x amps x 3.414/1.08 x temperature rise

Airflow checks using an electric furnace or air handler:

1. Set the thermostat selector to call for heat and the fan switch to "ON."

Note: Ensure that the fan is operating on the cooling speed.

2. Call for heat at the thermostat.

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3. When the indoor fan and all electric elements are on, measure the voltage to the fan and heaters.

4. Measure the amperage to the indoor fan and electric heaters with an ammeter.

5. Multiply the voltage times the amperage. This equals the wattage used to heat the air.

6. Divide the result by 1,000. This gives the kilowatts (kW) used.

7. Measure the temperature of the return air and the supply air leaving the air handler or electric furnace.

8. Subtract the returning air temperature from the supply air temperature. The result is the temperature rise over the heaters and fan motor.

9. Refer to Table 1 provided in the PDF link at the bottom of this page. Find the kW on the left side. Move across the chart to the temperature rise, as measured. Read the cfm above that column.

Note: This chart only applies to sea level airflow. Certain adjustments must be made if using at higher elevations to account for air density differences.

Example:
230 volts x 45 amps = 10,350 watts
10,350 divided by 1,000 = 10 kW
10 kW with a temperature rise of 26 degrees
From the chart, find 10 kW, over to 26, read up to 1,200 cfm
This airflow is correct for a 3-ton system (3 x 400 cfm/ton)

Airflow checks using a gas or oil furnace:

cfm = unit output (Btuh)/1.08 x temperature rise

1. Set the thermostat selector to call for heat and the fan switch to "on."

Note: Ensure that the fan is operating on the cooling speed.

2. Call for heat at the thermostat.

3. Measure the temperature in the supply air plenum in a location that is "out of line of sight" from the heat exchanger.

4. Allow the furnace to operate until this temperature stops rising.

5. Measure the return air temperature just entering the blower compartment.

6. Subtract the return air temperature from the supply air temperature. The result is the temperature rise.

7. Calculate the heating cfm using the above formula.

Example:
Gas Furnace
Input: 75,000 Btuh
Output: 60,000 Btuh

Supply air temperature: 130 degrees F
Return air temperature: 70 degrees F
Temperature rise: 60 degrees F

cfm = 60,000/1.08 x 60 degrees

cfm = 926

Note: Always clock the gas meter for the correct furnace input.

Reprinted with permission from the Split System Residential Air Conditioners service manual from Rheem Air Conditioning Division, Fort Smith, Ark. For more information, visit www.rheemac.com.

Publication date: 07/11/2005